The present invention relates to a water-in-oil type (hereinafter referred to as w/o type) emulsion explosive.
Since the first disclosure of a w/o type emulsion explosive in U.S. Pat. No. 3,161,551, the improvement thereof has been carried out and disclosed in U.S. Pat. Nos. 3,242,019; 3,447,978; 3,715,247; 3,770,522; 4,008,108 and 4,110,134. The w/o type emulsion explosives disclosed in the above-mentioned U.S. Patents are the explosives which contain fundamentally as the continuous phase thereof (oil component) such as a hydrophobic carbonaceous fuel, mineral oils, waxes and the like, as the discontinuous phase thereof, an aqeuous solution of an oxidant mainly composed of ammonium nitrate, and as an emulsifier, w/o type emulsifier, and by adding a sensitizer such as nitric acid, strontium ions, and hollow microspheres thereto at any time, a sensitivity ranging from Boosterinitiation to No. 6 cap can be obtained. It has been well known that the w/o type emulsion explosives have excellent waterproofness and safety, which have never been given in the conventional explosives, because they contain an oily substance as continuous phase and aqueous oxidizer solution as discontinuous phase.
However, since the essential feature of emulsification is to bring one of the two mutually-insoluble solutions into minute particles and to disperse the thus obtained minute particles into the other solution uniformly by an emulsifier, a shortage of being poor in stability has been observed in the w/o type emulsion explosive according to the inventions of the U.S. Patents. Namely, the w/o type emulsion explosives have the desired sensitivity and explosive power just after the production thereof, however, as the time passes by, the discontinuous phase which has been dispersed at first becomes aggregated and very large in particle size resulting in the break-down of the emulsion, and accordingly, there has been observed a problem in storage that the initial sensitivity and explosive power has been lost within a few months. In the case where the period from the production of the explosive to the use thereof is very short such as a few hours to a few days as in the so-called site mixing method or in the case where the explosive is used in a similar way to the former, the problem in storage is not so large, whereas, in the case where the period from the production of the explosive to the use thereof is from 6 months to about one year, the problem in storage that the initial sensitivity and explosive power is lost becomes a severe problem.
Accordingly, studies for improving a w/o type emulsion explosive to be better stability after storaging for a long period have been carried out, and as a result, an invention of improving the stability in storage of a w/o type emulsion explosive has been completed as in U.S. Pat. No. 4,386,977.
As a result of the present inventors' further studies for improving the stability of w/o type emulsion explosives in storage, it has been found by the present inventors that in the w/o type emulsion explosives according to the conventional inventions, the emulsion thereof become to be broken as time goes by, and the sensitivity thereof is reduced, and particularly, that even in the stable w/o type emulsion explosive disclosed in U.S. Pat. No. 4,386,977, the air-gap sensitivity of the explosive becomes reduced as time goes by, while the detonation velocity and the cap sensitivity are not reduced. In the cases where the explosives are actually used, there is almost no chance of using only one package of the explosive and usually, a few packages or, according to circumstances, ten and a few packages of the explosive are arranged parallel in a hole and fired simultaneously. In such cases, the deterioration of the air-gap sensitivity of the explosive as time goes by, causes a large problem.
As a result of the present inventors' studies and experiments for improving the shortage of the w/o type emulsion explosives, particularly the deterioration of air-gap sensitivity of the explosive as time goes by, it has been found by the present inventors that the use of a mixture of the oil component and at least one polymer selected from the group consisting of epoxy resin, unsaturated polyester resin, polybutene, polyisobutylene, petroleum resin, butadiene resin and ethylene vinyl acetate copolymer as the oily material of the continuous phase of the w/o type emulsion explosive remarkably reduces the degree of deterioration of the air-gap sensitivity of the explosive and also prevents the deterioration of the cap-initiation sensitivity and the explosion velocity.